Reaction of human sera with Eubacterium brachy: isolation and characterization of an extracellular antigen.

Recent studies have demonstrated an association of Eubacterium sp. with the subgingival microflora of patients with chronic periodontitis. One species, Eubacterium brachy, was evaluated to determine the possible mechanisms by which this microorganism may contribute to these diseases. Of 167 sera evaluated by double diffusion in agar, 20.8% displayed reactivity with a sonicated preparation of E. brachy. An extracellular antigen was identified in the culture supernatant fluid which reacted with antibodies in human sera. This antigen was isolated by methanol precipitation and purified by gel filtration. When tested by an enzyme-linked immunosorbent assay, all sera displayed some reactivity. Lines of identity were not shared with other species of Eubacterium, but were shared with other clinical isolates of E. brachy. The reactive antibody was identified as immunoglobulin G by immunoelectrophoresis, verified by enzyme-linked immunosorbent assay, and found to be capable of complement fixation. The monosaccharides and amino acids of the extracellular antigen were identified by high-pressure liquid chromatography. This antigen was shown to have a molecular weight of 170,000 and to share a line of identity with the sonicated preparation of E. brachy. The possible role of the organism in the immunopathology of periodontal diseases is discussed.